U.S. Pat. No. 4,021,326 Pollner et al describes using a porous overcoat on the exhaust electrode of a zirconia-type exhaust gas oxygen sensor. Such a coating not only prevents erosion of thin film exhaust electrodes but also can provide a greater residence time of exhaust gases on the exhaust electrode surface. Obviously, the overcoat should be of an abrasion-resistant material and should be porous. The aforementioned Pollner et al patent describes forming the porous overcoat by calcining a coating of raw oxides in situ, or by sintering a coating of the desired oxide material to the zirconia. It can also be applied by flame spraying, or plasma spraying. The aforementioned Pollner et al patent also describes applying the overcoat by thin layer techniques such as thermal evaporation, precipitation from gases, and reactive vapor deposition. U.S. Pat. No. 4,116,883 Rhodes describes applying a water slurry of very fine alumina particles and then merely drying the coating at 500.degree.-800.degree. C. Flame spraying has been commercially used to overcoat zirconia-type sensors. On the other hand, such a technique is expensive. The coating technique described in the aforementioned Rhodes patent has also been commercially used. However, this latter type of coating is primarily useful for catalyst support purposes. If a catalyst is not used in the overcoat, this latter coating may not be preferred. Also, I do not expect that it is as durable as a sintered or flame sprayed coating.
I have found a new method of forming a porous overcoat on a noble metal exhaust gas electrode of a zirconia-type exhaust gas oxygen sensor. My coating is formed inexpensively and yet provides high porosity and durability.